I. Field of the Invention
The present invention relates to internal combustion engines and, more particularly, to a valve guide construction for the intake or exhaust valve on such internal combustion engines.
II. Description of the Prior Art
In previously known internal combustion engines, and particularly aircraft reciprocating piston engines, the valve guide has been fabricated from niresist material (cast iron matrix) and installed in the valve guide bores in the cylinder head during construction of the cylinder head assembly. Once the guide is installed within the cylinder head, the guide is reamed to its final desired size. Other materials, such as tin bronze, aluminum bronze and gray iron have also been used to fabricate valve guides in a similar fashion.
This previously known method of reaming the valve guide to its final size after installation of the valve guide in the cylinder head was preferred because of the necessity of insuring concentricity and perpendicularity of the valve guide and thus the valve with the valve seat. Thus, by reaming the valve guide to its final size after installation in the cylinder head, all variations in dimension between the valve guide, the valve and the seat are automatically compensated for.
The materials previously used for the valve guide have been sufficiently ductile to permit mass machining of the valve guides and specifically the reaming of the valve guide bore after installation of the valve guide in the cylinder head. If a material harder than the niresist material were used for the valve guides, the manufacturing process of reaming the valve guide to its final size would likewise increase in difficulty and render it unsuitable for mass production.
The use of the relatively ductile materials for the valve guides, however, is disadvantageous in that the valve guides wear relatively rapidly during operation of the engine. Excessively worn valve guides result in high oil consumption due to leakage around the valve stem which likewise results in a smokey and oil laden exhaust. Similarly, excessive valve guide wear also causes excessive crankcase pressures with oil blowover.
Most importantly badly worn valve guides also result in burning of both the valve and its valve seat and thus weaken the exhaust valve stems due to improper valve seating. After prolonged engine operation, a worn valve seat results in premature cylinder replacement. Burnt exhaust valve seats can induce preignition with its catastrophic results thus necessitating a complete engine overhaul.